The present invention relates to a new and improved contact-controlled capacitive switching means operating in a non-contact or contactless signalling mode. Such switching means are utilized, for example, in combination with floor or cabin calling transmitters in lift controls or lift control systems.
Generally speaking, the switching means or apparatus of the present development is of the type comprising an oscillator including a resistor-capacitor element determining the oscillator frequency. There is also provided a switching circuit connected after the oscillator, the input of which is connected to the oscillator and the output of which is adapted to be connected to a terminal of a signal transmitting line. The switching means also comprises a coupling member comprising at least one resistor and having a first and a second terminal, the first terminal being connected to the capacitor in the oscillator and the second terminal being adapted to be connected to an actuator element, such as a contact button. The capacitor in the oscillator is grounded. The oscillator frequency is changed if the contact button is activated and there is brought about a change of a discrete signal present at the output of the switching circuit.
A known switching means of this kind, for instance as disclosed in Austrian Pat. No. 362,446, comprises an oscillator including a Schmitt trigger and a frequency-determining resistor-capacitor element. The switching circuit which is series connected to the oscillator is formed by three series connected monostable multivibrators. The coupling member comprises a capacitor series connected with a resistor or just a resistor. The capacitor of the frequency-determining resistor-capacitor element has one connection to ground, so that the coupling member is connected in parallel to the capacitor of the frequency-determining resistor-capacitor element if the contact button is touched or otherwise actuated.
When utilizing such switching means in combination with a floor or cabin calling transmitter in elevators or lifts, reasons of safety require that, for instance, in case of fire faulty operation of the calling transmitter is prevented. This requirement cannot be satisfied by the switching means as described hereinbefore. In the case of fire breaking out on one particular floor, the air may become ionized due to the action of heat, so that an electrically conductive connection is established between the contact button and ground. A lift or elevator cabin servicing such a call may, then, become blocked at the burning storey or floor, for example by failure of the door control. The same effects also may result from damage or destruction to any one of the input or output-end components of the switching means due to thermal action.